Like it was said way upthread... I see this as a manifestation of the Windows/Mac malware thing. All the bad guys are optimizing their dark SEO for Google, not Bing. If they decided to focus on Bing, given time to catch up to their extensive knowledge of Google internals, the same would happen to our bingy buddy.
A thought: does Google factor in domain registration time to its algorithm? This seems like a reasonably accurate heuristic for tracking original content vs. scrapers. Obviously a "reused" scraper domain would be the problem.

Let's look at where stackoverflow.com traffic came from for the year of 2010. When 88.2% of all traffic for your website comes from a single source, criticizing that single source feels … risky. And perhaps a bit churlish, like looking a gift horse in the mouth, or saying something derogato...